1. Field of the Invention
This invention relates to a projection screen, more particularly to an optical projection screen that has a relatively low light reflection with respect to ambient light, relatively high black-and-white contrast image, relatively high gain, and a half gain viewing angle of greater than 175 degrees.
2. Description of the Related Art
Referring to FIG. 1, a conventional projection screen 1 is usually used in a movie theater, a meeting room, a home theater, etc., and is able to diffuse and reflect a projector light 12 from a projector 11 to display an image thereon. When the projector light 12 goes in a constant direction with a constant luminous flux, a ratio of a maximum reflected luminance on the projection screen 1 to a standard luminance of the projector light 12 is defined as “maximum luminance coefficient” (i.e., again of the projection screen 1). Along a horizontal direction, the luminance coefficient or gain decreases from a center point on the projection screen 1. A half gain viewing angle refers to a viewing angle at which the reflective luminance becomes one-half of the maximum reflected luminance.
Because the conventional projection screen 1 is likely to reflect or diffuse ambient light, it is preferably used in a dark environment to generate a better visual effect. However, it is difficult for viewers to write in such condition when the projection screen 1 is used for teaching or in a meeting. Therefore, the conventional projection screen 1 needs further improvement.
Referring to FIG. 2, U.S. Pat. No. 6,747,796 discloses a projection screen 100 including a substrate 10 and a layer of transparent beads 13 coated on the substrate 10. The projector light passes through the transparent beads 13 and is refracted to the viewers. By virtue of the transparent beads 13, the gain of the projection screen 100 can be enhanced. However, since each of the transparent beads 13 has a spherical structure that is likely to refract the ambient light, the half gain viewing angle of the projection screen 100 ranges from 40 degrees to 70 degrees. The viewable angle of the projection screen 100 is too small for use in a place where many people are present.
Furthermore, while the conventional projection screens 1, 100 may reflect or refract ambient light, they can not function to enhance color saturation and black-and-white contrast. Therefore, images displayed by the conventional projection screens 1, 100 have poor color saturation and black-and-white contrast.